1. Field of the Invention
This invention relates to an apparatus for molecular beam epitaxial growth of semiconductor crystals.
2. Description of the Prior Art
In recent years, attention has been paid to molecular beam epitaxy (MBE) as a high crystal growth technique using the quantum effect for the production of devices such as quantum-well semiconductor lasers and high-electron-mobility transistors (HEMT). Molecular beam epitaxy produces growth layers with a thickness that can be controlled more accurately to the level of layers of molecules than liquid phase epitaxy (LPE). Moreover, the growth layers obtained by molecular beam epitaxy are excellent in the uniformity of the thickness distribution on the surface thereof, so that semiconductor devices having excellent characteristics can be produced in a high yield.
However, in order to obtain crystals of good quality by molecular beam epitaxy, molecular beam intensity must be controlled with accuracy. For example, in the growth of crystals for semiconductors with elements in the range of Groups III to V or Groups II to VI of the periodic table, when both the vapor pressure and the substrate temperature are high, many problems arise with the control of the elements in Groups V and VI both of which have a low effective adhesion coefficients for the substrate. For example, in the growth of InP, when the temperature of the substrate is high, the P revaporizes, and the P pressure must be raised. However, when the P pressure of the growth chamber in which the epitaxial growth is being carried out is too high, there are problems in maintaining the growth chamber at a perfect vacuum. For this reason, substrate temperatures generally used are under 600.degree. C. When a solid source of P is used, a molecular beam of P.sub.4 molecules is employed. At such low substrate temperatures, a high sticking efficiency of the P.sub.4 molecules to the surface of the substrate cannot be achieved, so that good quality crystals cannot be obtained. Thus, a P.sub.4 molecular beam once obtained passes through a cell containing a decomposing furnace in which the P.sub.4 molecular beam is converted to a P.sub.2 molecular beam by passage through the high heat of a furnace, in the range of about 800.degree.-1000.degree. C.
On the other hand, in crystals containing Al, such as GaAlAs, increasing the substrate temperature attains better crystallinity. Thus, since the substrate temperature cannot be raised for crystals containing both Al and P such as InAlP, good quality crystals cannot be obtained. Moreover, when the cell attached to a high temperature decomposing furnace is used, new contamination problems may be caused by at least tantalum and carbon, which are contained in the furnace materials, due to the high temperature of the decomposing furnace. Moreover, there are various other drawbacks such as difficulty of temperature control of the decomposing furnace and difficulty in setting up the apparatus required for heat-insulation between the high temperature decomposing furnace zone and the low temperature solid source zone.